Response correction for SAW filters

ABSTRACT

A procedure to correct the amplitude of the frequency or the impulse response for signal processing filter based entirely or partly on acoustic surface waves has been described. The method may be applied where the acoustic wave follows different paths inside the component depending on the signal frequency or on the signal time delay in the filter. The method is based on changing the signal phase in an essentially periodic way for instance by etching metal films placed on the surface of the component in order to make correction possible. Laser induced etching can be an efficient way to carry out amplitude correction according to the invention due to the good geometric resolution of this method and because correction of the phase pan of the response can be earned out with the same equipement in accordance with established techniques.

The present invention relates to the correction of the amplituderesponse in the time domain or in the frequency domain for a class oflinear, reciprocal filters which are based on surface acoustic waves(SAW). The signal is carried in the form of a SAW over the surface ofthe component. The SAW will follow a path from an input transducer wherethe SAWs are generated to an output transducer where the SAWs aredetected and the signal is made available to the user in electric form.Only a part of each signal carrying SAW is converted to electric form bythe output transducer; such part will be referred to part of the usefulacoustic signal or saw.

Existing correction methods are based on the fact that the usefulacoustic signal follows different paths over the component's surfacedetermined by the signal frequency if the correction is to be made inthe frequency domain or dependent on the signal time delay if thecorrection is made in the time domain.

Correction of the phase response for SAW filter has been done and isbeing done today by the deposition of a metal film which influences thevelocity of the SAW. After electrical measurement of the component'sresponse, part of this metal film may be removed by a deliberateselective etching until a correct phase response has been reached asdescribed by V. S. Dolat, J. H. C. Sedlacek and D. J. Ehrlich in thepaper "Laser direct write compensation of reflective array compressors",IEEE 1987 Ultrasonic Symposium Proceedings, p. 203-208.

Correction of the amplitude response for reflector array compressor(RAC) chirp filters has been done earlier by the deposition of aresistance film (CERMET) which will attenuate the SAW as described inthe above Dolat et al. paper. After electrical measurement of theresponse of the RAC filter, the attenuating film has been made partiallyinactive by laser induced oxidation until a correct amplitude responsehas been reached. This method requires a strong piezo-electric couplingfor the waves in the area where the film is located. This is notpossible for RAC chirp lines on quartz, in particular not if the crystalhas a so-called ST-cut. As described by M. B. Schulz, B. J. Matsingerand M. G. Holland in the paper "Temperature dependence of surfaceacoustic wave velocity on alpha-quartz"; J. Appl. Phys, Vol 41,2755-2765 orientation of the SAW plane related to the crystal axisresults in a low temperature dependence of the component, but thecrystal cut lacks piezo-electric coupling for SAWs in the transversaldirection where the attenuating CERMET film has to be located. Other SAWmaterials like LiNbO₃ has a sufficient piezo-electric coupling, however,it represents a general problem to produce a CERMET film with a suitableresistance and a sufficient stability.

By using the invention, correction of the amplitude response on a timeselective or a frequency selective basis is obtained by changing thesubstrate surface properties in a deliberate pattern transversely to thepropagation of the wave to obtain desired SAW velocity changes. Thesevelocity changes have to be made in that area of the surface where thesignal path depends on frequency or on time delay. The resulting changein the transfer of the useful signal can be used for amplitudecorrection of the filter response by a procedure of the presentinvention that will be described below.

A RAC filter, or in principle any other filter, may also be corrected bymaking a cascade connection to a correction filter which has beendesigned to cancel the response errors of the RAC filter to becorrected. This method is described in the U.S. Pat. No. 4,857,870:"Method of manufacturing a surface wave dispersive filter and a filtermanufactured in accordance with this method". The additional filter, orthe correction filter can be integrated with the primary filter on thesame piezo-electric substrate or on an additional substrate dependent onwhat is the most practical and economic solution. In any case thecorrection of a filter by the design of a particular correction filterwill be a rather expensive method.

One of the other amplitude correction methods is to correct the depthsof the reflector grooves after the component has been produced andelectrical measurements of the response made. This procedure requires apermanent masking of the ridges between the grooves. This mask serves asan etching mask for the correction etching of the grating withoutdestroying the propagation properties of the SAWs in the reflectivearray. This procedure requires a high geometric resolution for theetching process along the reflector arrays. An adequate resolution canbe difficult to obtain for chirp lines with a high chirp rate, that is asmall change in the time delay per unit frequency. The masking of theridges has disadvantages in that it may result in additional losses andalso be a possible source of accelerated aging of the component. It hasalso been difficult to find suitable material/etch process combinationsfor an attractive SAW substrate material like LiNbO₃.

It is therefore desireable to develop new methods for amplitudecorrection. For economic reasons it would be preferable to use the sametechnology and the same equipment for the amplitude correction as isused for the phase correction of the response. This property is obtainedby the inventive amplitude correction method.

The invention and the advantages obtained with the invention will bemore fully appreciated from the following detailed description when thesame is considered in connection with the accompanying drawings, inwhich

FIG. 1 schematically and in principle shows a reflector array compressor"RAC" surface acoustic wave "SAW" chirp filter.

FIG. 2 schematically and in principle shows a reflector dot array "RDA"filter.

The principal function of a RAC filter as shown in FIG. 1 is that theinput transducer 1 sends a SAW wave towards an array 2 of reflectinggrooves which reflects the waves with a 90° change of direction. Thehigh frequency signals are reflected close to the input transducer 1 andlow frequency signal further back into the array. This effect isobtained by increasing the distance between the grooves as one goesfurther away from the input transducer. The waves propagate towards another reflecting array 3 which is the mirror image of the first arrayaround the line denoted x--x in FIG. 1. This second array reflects thewave towards the output transducer as described by A. Ronnekleiv in thepaper "Amplitude and phase compensation of RAC-type chirp lines onquartz", IEEE 1988 Ultrasonic Symposium Proceedings, pp. 169-173.Signals with different frequency will pass the line x--x at differentlocations and they will also be subject to different delays through thecomponent. The frequency dependent delay is the primary function of achirp filter.

The "U-structure RDA"-filter as described by L. P. Solie in the paper "ASAW filter using a reflective dot array (RDA)", IEEE 1976 UltrasonicsSymposium Proceedings, pp. 309-312, is shown in FIG. 2. One differencefrom the RAC-chirp filter in FIG. 1 is that the individual reflectors inthe arrays 2 and 3 instead of being made as grooves, are made as astring of metal dots placed on a straight line. As related to theinvention it is a more interesting difference that the reflectors ofFIG. 2 are equidistant along the arrays 2 and 3. This implies that theyare all effective reflectors in the same frequency range. Hence for theRDA filter structure the signals which pass a line x--x will have thesame frequency range, but obtain different time delays dependent on thelocation where the signals pass the line x--x.

The invention concerns only correction of the amplitude response. Wewill now establish a relation between phase correction of RAC filters byusing existing technology and the inventive method for amplitudecorrection. To this end we introduce a periodic change of velocity forthe waves along the line x--x in the area between the arrays 2 and 3.The period of velocity change is L_(p) and it has a sine shaped functionalong the line x--x shown between the arrays 2 and 3 in FIG. 2. Inaccordance with the aforementioned Volck et al. paper, this change ofvelocity will result in a corresponding change of phase for the wavespassing this area. Let W denote the width of each of the arrays measuredorthogonal to the line x--x. If L_(p) ≧2 W the result will be a periodicchange in the phase of the impulse response for the fighter (periodic intime), with period T_(p) =L_(p) /(2 v₁) where v₁ is the wave velocitymeasured along the arrays 2 and 3 in the x--x direction. If L_(p) ismade sufficiently large the result will be a change of phase in thefrequency response of the filter. This case corresponds to the wellknown method for phase correction of a RAC filter response. Theamplitude change of the impulse response will be negligible.

At or close to the range 2 W≧L_(p) ≧W/2, the magnitude of the resultingperiodic change of phase for the impulse response will be reduced as wereduce L_(p). For L_(p) =W/2 the change of phase for the impulseresponse will be nearly constant and independent of the time delay, andapproximately equal to the average value of the phase change measuredalong the line x--x. The amplitude of the impulse response when L_(p) isreduced from 2 W will be reduced and stabilized at a level given by theamplitude of the periodic phase change when L_(p) is reduced to W/2. Afurther reduction in L_(p) will not affect the amplitude nor the phaseof the impulse response in a significant way before L_(p) is reduced toa size comparable to the wave-length λ. If L_(p) <λ the wave propagationwill be that of a homogeneous medium. Hence no periodic phase changewill occur in the output signal and the amplitude response will assumeits original value. The phase change of the impulse response willcontinue to be constant as a function of the signal time delay but mayvary with L_(p) for L_(p) ≈λ or shorter.

The amplitude change for the impulse response of the filter which occursat a periodic velocity change and which mainly occurs for W/2≧L_(p) ≧λ,is the basis of the inventive amplitude correction method. This effecthas not been described earlier and hence not suggested used to correctamplitude response. The above V. S. Dolat et al. paper describes phasechanges over short isolated areas along the line x--x and observes thatthese phase changes can be used for phase correction and gives a smalllocal amplitude change. This paper though, does not observe that theamplitude changes can be made uniform over large areas of time delay forthe impulse response or large frequency ranges of the frequency responseby introducing periodic phase changes with W/2≧L_(p) ≧λ along the linex--x of FIG. 1, and they therefore fail to observe that these effectscan be attractive also for amplitude correction of RAC filters. Thepossibility for amplitude correction can be seen from the mode couplingintegrals which are to be described in the following text, equations 1and 2. This formalism has not been proposed in the Dolat et al. paper.

To explain the amplitude correction method applied in the frequencydomain, let us consider the signal amplitude along a curve withcoordinate x transverse to, but not necessarily orthogonal to thedirection of SAW propagation in the area where the useful signal willfollow different paths dependent of the frequency. The line x--x in FIG.1 will be suitable for this purpose. At a given signal frequency let usassume that an electrical signal applied to transducer 1 gives a waveamplitude a(x) along the curve and that a signal applied to thetransducer 4 gives a wave amplitude b(x) along the same curve with xdenoting the distance from the transducer ends of the arrays 2 and 3.The functions a(x) and b(x) are functions with complex values where theabsolute value represents the amplitude of the wave and the phase of thefunction represents the phase of the wave. The expression ##EQU1##represents a mode coupling loss between the two transducers 1 and 4. Theintroduction of a velocity change for the useful SAWs in the vicinity ofthe curve x will change the mode coupling loss. If L_(p) is largecompared to λ, the effect on the complex wave amplitude will simply be aperiodic change in the phase as the waves cross the perturbed area,φ(x), and the mode coupling loss will now become ##EQU2## whereexp(jφ(x)) is the exponential function of jφ(x) and j=√-1.

The transmission through the component has now been changed with theratio |A₂ /A₁ | in amplitude. By making a(x)=[b(x)exp(jφ))]* (* denotedthe complex conjugate), the maximum response of the component isobtained. To reach exactly this value is usually of less interestbecause the functions a(x) and b(x) are difficult to find in detail andit is hardly possible to maximize the response at several or allfrequencies at the same time. On the other hand if φ(x) has a rapidvariation compared to a(x) and b(x) and a mainly periodic variation,φ(x) may be used to reduce |A₂ A₁ | to obtain a desired amplitudecorrection. The period of L_(p) should not be made less thanapproximately one wavelength in the x direction because, for the wavepropagation perturbation with such a periodicity, the waves will mainlysee a local average value of the propagation parameters. The inducedsurface perturbations will therefore not show up as phase changes of thewaves, and the effective φ(x) will be close to zero and the lossesdescribed by eq. 2 and intended to be used for amplitude correctionaccording to the invention will disappear.

The description above indicates that the phase changes occurs in anarrow belt transverse to the wave propagation direction. This is not anecessity according to the invention in that a wider structure in thedirection of the signal propagation can be split into narrow belts whichare numbered from 1 to n. Each of these belts allows an independentapplication of eq. 2 to find a desired effect of the phase change φ_(n)(x) in belt No. n as a factor of (A₂ /A₁)_(n), where the transmissionproperties for the belts 1 to n-1 has been included in the computationof the related attenuation functions. The total correction will be theproduct of all N sequential corrections. It is obvious that theamplitude correction can be different for two different frequencies onlyto the degree that the useful SAW follows two different paths throughthe component or alternatively are influenced in different ways by theSAW material perturbation in an area common to the signals at the twodifferent frequencies.

It is of practical interest to observe that if for two subsequentcorrections for the same component φ(x)=φ₁ (x)+φ₂ (x) where φ₁ (x) andφ₂ (x) both are periodic functions but the functions have no commonharmonics (in particular important for the lower harmonics), the twocontributions to the amplified reduction factors (see eq. 2) will beindependent of each other.

We have so far discussed amplitude correction of the frequency responseof a component. In principal the same procedure can be used foramplitude correction of the impulse response of a component. For animpulse response amplitude correction it is required that signals whichhave different delays through the components will pass the component bymore or less different paths.

To obtain a good correction for the impulse response it is preferablethat the total frequency response of the component is relativelynarrowband and that the major band limitation occurs outside the areawhere the correction is carried out. To obtain a fairly simpledescription of the method, band limited signal will be assumed whichmeans that the phase change of the signal in the correction area isreasonably well defined. Once again let x be a coordinate transverse tothe wave propagation direction, and let y be a coordinate along the wavedirection. a(x, y, τ) is the complex wave amplitude caused by an impulseof the input coupler 1 after a time τ and b(x, y, τ) the correspondingamplitude caused by an impulse to the output coupler 4 in FIG. 2. Inthis case the impulse response of the component at a time τ will beproportional to, wherein n is a time parameter which for a givencontribution to the convolution integral (3) gives the time delay seenby the wave through the impulse response a(x,y,η)

    ∫∫∫a(x,y,η)b(x,y,τ-η)dxdydη (3)

It is a consequence of equation 3 that by imposing a variable phaseshift φ(x) for the waves around a given y-coordinate of the material itis possible to influence the amplitude of the impulse response here inthe same way as described above for the frequency response. Amplitudecorrection of the impulse response is therefore possible.

For those cases where a description with a frequency independent phaseshift function φ(x) for the correction is too coarse, it is possible touse a more exact description of the wave propagation effects of thechanges based on time delay or frequency dispersive time delay. Also forthis more detailed description the effect on amplitude may be found withgood accuracy from expressions of the form shown in eq. 3. If theimpulse response correction has to be accurate over a wide frequencyband, the frequency spectrum of the time delays used when weighted withthe amplitude of a and b according to eq. 3, should give a flatfrequency response across the desired band at the same time as itreduces the amplitude in a desirable way.

FIG. 2, in which the same reference numerals as before have been used todenote corresponding parts: shows a "U"-structure "RDA"-filter that iswell suited for amplitude correction of the impulse response accordingto the invention rather than frequency response.

The local changes of the velocity which must be introduced in the wavepropagation to use the amplitude correction methods described above, maybe obtained by placing thin films of metal or other suitable materialson the surface of the component in the area suited for correction. Thefilms will change the wave velocity by shortening electric fieldsgenerated due to piezo-electric coupling and/or by loading the surfacemechanically. The films may be patterned by standard photolithographictechniques. It is of interest to note that as noted in theaforementioned Dolat et al. paper, molybdenum, Mo, and some othermaterials (Si, Ti) may be etched by laser induced processes to obtain adesired change of the pattern without disassembly and wet processing ofthe components. It is also possible and from an acoustic point of viewequally effective to deposit patterned films either by the usualphotolithographic methods or by locally stimulating deposition of a filmfrom a gas by laser light. Several applications of this technique aredescribed by D. Bauerle in the book "Chemical Processing with Lasers",Volume 1 of Springers series in Material Sciences. (ISBN 3-540-17147 -9,Springer, Berlin, 1986).

The amplitude correction according to the invention will result inundesirable changes of the signal phase. The practical problemassociated with this is minor, because the phase can be corrected byestablished techniques such as those disclosed in the above Dolat et al.paper, which may be carried out with the same production equipment asbefore.

I claim:
 1. An analog surface acoustic wave filter, comprising:a surfaceacoustic wave propagating substrate; means including an input transduceron said substrate for launching a surface acoustic wave signal into saidsubstrate for propagation therethrough in response to an electricalsignal applies thereto; means including an output transducer on saidsubstrate for receiving at least a part of the surface acoustic wavesignal therefrom; means on said substrate for causing constituentsurface acoustic waves of said surface acoustic wave signal to propagatetoward said output transducer in a plurality of different paths whichare dependent on frequency, each of said paths passing through apredetermined area of the substrate and having at least a portion of apredetermined width, each said predetermined width being the same forall of said different paths; and means on said substrate for introducinga controlled deviation, with substantially periodic variation over saidpredetermined area of said surface, of at least one parameter thatdetermines a uniform surface phase velocity for the surface acousticwave signal, said controlled deviation varying, in a directiontransverse to said different paths, with a period that is at least equalto a wavelength of the acoustic wave signal and at most equal to thepredetermined width.
 2. An analog surface acoustic wave filter,comprising:a surface acoustic wave propagating substrate; meansincluding an input transducer on said substrate for launching a surfaceacoustic wave signal into said substrate for propagation therethrough inresponse to an electrical signal applied thereto; means including anoutput transducer on said substrate for receiving at least a part of thesurface acoustic wave signal therefrom; means on said substrate forcausing constituent surface acoustic waves of said surface acoustic wavesignal to propagate toward said output transducer in a plurality ofdifferent paths which are dependent on time delay, each of said pathspassing through a predetermined area of the substrate and having atleast a portion of a predetermined width, each said predetermined widthbeing the same for all of said different paths; and means on saidsubstrate for introducing a controlled deviation, with substantiallyperiodic variation over said predetermined area of said surface, of atleast one parameter that determines a uniform surface phase velocity forthe surface acoustic wave signal, said controlled deviation varying, ina direction transverse to said different paths, with a period that is atleast equal to a wavelength of the acoustic wave signal and at mostequal to the predetermined width.
 3. A procedure to correct theamplitude part of the frequency response for analog filters, each filtercomprising a surface acoustic wave propagating substrate, an inputtransducer for launching a surface acoustic wave signal into thesubstrate in response to an electrical signal applied thereto, and anoutput transducer for receiving at least a part of the surface acousticwave signal propagated through the substrate from said input transducerafter said surface acoustic wave signal part has followed a plurality ofdifferent paths through a predetermined area of the substrate dependenton frequency, each of said different paths having a predetermined width,characterized by the step of introducing a controlled deviation, withsubstantially periodic variation over the predetermined area of thesubstrate, of at least one parameter that determines a uniform surfacephase velocity for the surface acoustic wave signal, said controlleddeviation varying, in a direction transverse to said different paths,with a period that is at least equal to a wavelength of the acousticwave signal and at most equal to the predetermined width of thedifferent wave paths.
 4. A procedure to correct the amplitude part ofthe impulse response for analog filters, each, filter comprising asurface acoustic wave propagating substrate, an input transducer forlaunching a surface acoustic wave signal into the substrate in responseto an electrical signal applied thereto, and an output transducer forreceiving at least a part of the surface acoustic wave signal propagatedthrough the substrate from said input transducer after said surfaceacoustic wave signal part has followed a plurality of different pathsthrough a predetermined area of the substrate dependent on time delay,each of said different paths having a predetermined width, characterizedby the step of introducing a controlled deviation, with substantiallyperiodic variation over the predetermined area of the substrate, of atleast one parameter that determines a uniform surface phase velocity forthe surface acoustic wave signal, said controlled deviation varying, ina direction transverse to said different paths, with a period that is atleast equal to a wavelength of the acoustic wave signal and at mostequal to the predetermined width of the different wave paths.